(a) Field of the Invention
This invention relates to a composite, oriented polypropylene film which has: a first alpha-polyolefinic skin layer providing a surface which releases easily from a cold seal receptive surface as well as from a cold seal cohesive composition adherent to the receptive surface on the opposite side of the film, such easy release surface is also simply referred to herein as a cold seal release surface; a second alpha-polyolefinic layer on the other side of the film which has a treated surface to which a cold seal cohesive composition is applied, such second surface also referred to herein as a cold seal receptive surface; and a third alpha-polyolefinic layer which is a core layer predominantly of isotactic polypropylene homopolymer. This film is useful in the packaging of articles such as confections.
(b) Background and Prior Art
Cold seal cohesives are generally natural or synthetic rubber latex compositions which when applied to a flexible packaging substrate allow the package to be cohesively sealed by pressure and dwell time at ambient temperature. Cold seal cohesives are generally applied to the inside (cold seal receptive surface) of a film lamination or to a single web film. In each case, the outer film surface (facing away form the cold seal) must repel adhesion to the cold seal on the inner web in order that the roll stock may be unwound when it is used to wrap the product. In the case of single web film, the surface opposing the cold seal is typically coated with a polyamide type overlaquer in order to provide sufficient release from the cold seal, i.e., prevent roll blocking. In the case of laminated films, the outer web of the film is generally modified with migratory additives which when sufficiently bloomed to the surface promote low coefficient of friction (C.O.F.) for machinability and cold seal release (C.S.R.). It is known that migratory additives, particularly amides, coming in intimate contact with the cold seal composition surface in sufficient quantities will cause the cold seal composition cohesive strength to become significantly weakened. This phenomenon is known as cold seal deadening.
Biaxially oriented polypropylene (B.O.P.P.) film by itself does not give adequate cold seal release (C.S.R.) or coefficient of friction (C.O.F.) and requires the addition of additives to accomplish these objectives. These additives, which are predominantly migratory, have two main problems: (1) they must bloom to the film's surface and remain there in order to be consistently effective; and (2) they have a tendency to retard the cold seal cohesive strength. The current industry standard film for this application is a mono-layer B.O.P.P. of isotactic polypropylene homopolymer of Hercules, Inc. containing a migratory amide slip agent and which is designated T 523 or T 522. In the case of T 523, the slip modified film is corona discharge treated to an equal level of both surfaces and, as such, can be used on either side. In the case of T 522, the film is corona discharge treated on only one side. The untreated surface is the preferred surface for cold seal release. These films, however, need to be aged for a period of time after production at a certain temperature so that the cold seal release and C.O.F. additives sufficiently bloom to the surface for the film to be functional.
The present invention is to a single web polypropylene film which can be reverse printed and/or pattern cold seal coated on the cold seal receptive surface and released from the opposing film surface without the need for a cold seal release overprint varnish and the migratory slip agents which deaden the cold seal composition. This eliminates a step in the converting process as well as reducing the overall product costs. This is made possible by a polymeric film surface on one side of the core which gives excellent cold seal release without relying on conventional migratory slip and antiblock additives as well as a surface treated polymeric film layer on the other side of the core which provides a substrate for excellent adhesion of the cold seal composition.
The inventive film demonstrates:
A. Good to excellent coefficient of friction when the film with or without the cold seal composition is unwound from a roll; PA1 B. Good to excellent adhesion of the cold seal composition on the cold seal receptive surface. PA1 C. Excellent optics; PA1 D. Good printability on the cold seal receptive surface; PA1 E. Good machinability and stiffness typical of oriented polypropylene film; and PA1 F. Absence of poisoning (deadening) the cold seal cohesive properties. PA1 1. Immediate and permanent release of the cold seal receptive surface as well as the cold seal composition from the surface of the cold seal release layer on the other side of the film at the time of manufacture and roll winding of the film. Thus, no film aging is required and therefore the conventional overprint varnish for single web polypropylene films is not required. Elimination of the overprint varnish saves on converting time and costs. PA1 2. Permanently good cold seal receptivity on the cold seal receptive surface. PA1 3. Permanently low coefficient of friction on unwinding a roll of the film due to the absence of migratory slip additives. PA1 4. No cold seal deadening because it does not possess migratory additives in type or quantity do so. PA1 5. The cold seal release surface will readily accept code dating inks.
The present invention has the following principal advantages over the prior art: